Progress has been made with higher sensitivity and smaller format silver halide color negative films in recent years, and there is a demand for color negative photographic materials which have high sensitivities and provide excellent image quality.
Thus, the requirements of silver halide emulsions for photographic purposes have become more rigorous and a demand has arisen for a higher level of photographic performance in terms of high sensitivity, high contrast, superior graininess and sharpness, etc.
Techniques involving the use of tabular grains intended to provide an increase in sensitivity, including an increase in color sensitizing efficiency with sensitizing dyes, to improve of the relationship between sensitivity and graininess, to improve sharpness, and to increase covering power have been disclosed in response to these demands in U.S. Pat. Nos. 4,434,226, 4,414,310, 4,433,048, 4,414,306 and 4,459,353.
Furthermore, multi-layer color photographic materials which have a high sensitivity and improved graininess, sharpness and color reproduction in which tabular silver halide emulsions in which the grains have an aspect ratio of at least 8 : 1 are used in the high sensitive layers have been disclosed in JP-A-58-113930, JP-A-58-113934 and JP-A-59-119350. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
According to these patents, tabular grains improve sharpness when they are used in the blue sensitive layer because of their low scattering properties, and it has also been noted that graniness is improved when tabular grains are used in the green or red sensitive layers.
Moreover, multi-layer color photographic materials in which tabular silver halide emulsions in which the grains have an aspect ratio of at least 5 : 1 are used in the high sensitive layers and monodisperse silver halide emulsions are used in the low sensitive layers, and which have improved sharpness and color reproduction, have been disclosed in JP-A-61-77847.
Furthermore, methods in which the reflection of light which photosensitizes the emulsion in the layer above a layer in which tabular grains are being used is increased, and the sensitivity of the upper layer is increased by adjusting the thickness of the tabular grains, and in which reflection is minimized and there is no loss of sharpness of the upper layer, have been disclosed in Research Disclosure No. 25330.
Thus, tabular grains which have a high aspect ratio provide various advantages, but in the so-called sequential layer structure (a layer structure consisting sequentially starting furthest away from the support of blue sensitive layers, green sensitive layers and red sensitive layers of different sensitivities) which is most often used in color photographic materials, there is a problem in that the sharpness on the low frequency side deteriorates when tabular grains which have a high aspect ratio (for example an aspect ratio of 8 or above) are used in layers other than the photosensitive layer which is furthest from the support, and especially in the green or red sensitive layer.
Silver halide color photographic materials in which there is at least one blue sensitive layer which contains tabular silver halide grains of which the mean aspect ratio is at least 5, and in which substantially monodisperse, non-tabular silver halide grains of which the aspect ratio is not more than 5 are included in at least one of the green sensitive emulsion and red sensitive emulsion layers, have been proposed in Japanese Patent Application No. 61-235763 (corresponding to JP-A63-89839) as a means of ameliorating this deterioration of the sharpness of the green and red sensitive layers.
In this case, the use of regular crystals, such as tetradecahedral grains, is especially desirable for the monodisperse grains of which the mean aspect ratio is not more than 5. However, silver halide emulsions of which the distinguishing features are that, for the preferred grains, at least 70% of the total projected area of the silver halide grains is provided by tabular grains having a diameter of at least 15 .mu.m, that the aspect ratio of the said tabular grains is not more than 8.0, and that grains of which the ratio (b/a) of the longest distance (a) between two or more of the parallel twinning planes of the said tabular grains and the thickness of the grain (b) has a value of at last 5 account for at least 50% (by number) of all the tabular grains, and the use of the silver halide emulsions in at least one silver halide emulsio layer other than a silver halide emulsion layer which is located furthest from a support have been disclosed in Japanese Patent Application No. 61-311130 (corresponding to EP-A-0273411). According to Japanese Patent Application No. 61-311130, silver halide emulsions of which the sensitivity with respect to the average size is high when compared to regular crystals and with which there is no deterioration of graininess with respect to the average size are obtained by making the iodine distribution, the thickness (b) of the grains and the b/a distribution, and the grain size distribution uniform between the tabular grains, which are multiple parallel twin silver halide grains which have re-entrant angles which it has long been suggested form sites which can easily form chemical sensitization nuclei. Furthermore, although there is a problem in that the sharpness on the low frequency side deteriorates when a silver halide emulsion containing grains having the aspect ratio of 8 or above is used in a silver halide emulsion layer other than a silver halide emulsion layer located furthest from a support, when the silver halide emulsion as described in Japanese Patent Application No. 61-311130 is used, there is not the problem.
However, as a result of further investigation it has been discovered that these are unsuitable since pronounced fogging occurs when a pressure is applied to a silver halide photographic material which contains the said grains. Furthermore, a higher level is also required in respect of exposure luminance dependence (which is connected with sensitivity) and storage properties, etc., and the materials were still unsatisfactory from this point of view.
Techniques for improving pressure characteristics by establishing phases which have different average iodide contents within the silver halide have been disclosed in JP-A-59-99433 and JP-A-60-147727, but it has not been possible to achieve satisfactory pressure characteristics for practical purposes in this way.
A technique for improving pressure characteristics, and exposure luminance dependence and storage properties, by the controlled introduction of dislocations during the formation of the grains has been disclosed in Japanese Patent Application No. 62-54640 (corresponding to EP-A-0282896). However, although examples of tabular grains of which the aspect ratio is at least 2 are disclosed in the said patent there is no disclosure concerning the ratio b/a.
The inventors have therefore attempted to develop emulsions which have an excellent sensitivity/graininess ratio, and which also have excellent pressure characteristics, exposure luminance dependence and storage properties, by the controlled introduction of dislocations into tabular grains of aspect ratio not more than 8.0 of which the size distribution is uniform, of which the iodide distribution is uniform between the grains, and of which the longest distance (a) between two or more parallel twinning planes, the grain thickness (b), and the ratio b/a, are uniform. Further, the inventors have atempted to develop photographic materials which have excellent sensitivity/graininess ratio, sharpness, pressure characteristics, exposure luminance dependence and storage properties, by using the emulsion in a silver halide emulsion layer other than a silver halide emulsion layer which is located furthest from a support.